Operation of clamping retainer for chemical mechanical polishing

ABSTRACT

A method of polishing includes bringing a substrate into contact with a polishing pad and generating relative motion between the substrate and the polishing pad, retaining the substrate on the polishing pad with a retainer, and during polishing of the substrate alternating between reducing a diameter of an inner surface of the retainer to clamp the substrate and increasing the diameter of the inner surface of the retainer to release the substrate from clamping while continuing to retain the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. Application No. 63/346,802, filed on May 27, 2022, the contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to a retainer for use in chemical mechanical polishing of substrates and a method of operating such a retainer.

BACKGROUND

An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive, or insulative layers on a silicon wafer. One fabrication step involves depositing a filler layer over a non-planar surface and planarizing the filler layer. For certain applications, the filler layer is planarized until the top surface of a patterned layer is exposed. A conductive filler layer, for example, can be deposited on a patterned insulative layer to fill the trenches or holes in the insulative layer. After planarization, the portions of the conductive layer remaining between the raised pattern of the insulative layer form vias, plugs, and lines that provide conductive paths between thin film circuits on the substrate. For other applications, such as oxide polishing, the filler layer is planarized until a predetermined thickness is left over the non planar surface. In addition, planarization of the substrate surface is usually required for photolithography.

Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is typically placed against a rotating polishing pad. The carrier head provides a controllable load on the substrate to push it against the polishing pad. An abrasive polishing slurry is typically supplied to the surface of the polishing pad.

The carrier head provides a controllable load on the substrate to push it against the polishing pad. A retaining ring is used to hold the substrate in place below the carrier head during polishing. Some carrier heads apply pressure to urge the retaining ring into contact with the polishing surface.

SUMMARY

In one aspect, a carrier head for chemical mechanical polishing includes a housing, a substrate mounting surface, and a retaining ring assembly. The retaining ring assembly includes an inner ring surrounding the substrate mounting surface and having an inner surface to retain the substrate below the substrate mounting surface, an outer ring surrounding the inner ring, and an actuator positioned between the inner ring and the outer ring. The inner ring has a lower surface and a plurality of slots that are formed in the lower surface and that extend from the inner surface to an outer surface of the inner ring to divide the inner ring into a plurality of arcuate segments suspended from an upper portion. The actuator applies a radially inward pressure such that the plurality of arcuate segments flex inwardly relative to the upper portion.

In another aspect, a method of polishing includes bringing a substrate into contact with a polishing pad and generating relative motion between the substrate and the polishing pad, retaining the substrate on the polishing pad with a retainer, and during polishing of the substrate alternating between reducing a diameter of an inner surface of the retainer to clamp the substrate and increasing the diameter of the inner surface of the retainer to release the substrate from clamping while continuing to retain the substrate.

In another aspect, a polishing system includes a support to hold a polishing pad, a carrier head to hold the substrate against the polishing pad, and a controller. The carrier head includes a first chamber to apply a first downward pressure to a center portion of the substrate held by carrier head, a second chamber to apply a second downward pressure to an outer portion of the substrate surrounding the central portion, and an inner surface to engage an edge of the substrate. The inner surface has an adjustable diameter. The controller is configured to, in response to identifying a polishing non-uniformity, decrease the diameter of the inner surface of the retainer and select whether the first pressure is greater or lower than the second pressure so as to reduce the polishing non-uniformity.

In another aspect, a polishing system includes a support to hold a polishing pad, a carrier head to hold the substrate against the polishing pad, and a controller. The carrier head includes a first chamber to apply a first downward pressure to a center portion of the substrate held by carrier head, a second chamber to apply a second downward pressure to an outer portion of the substrate surrounding the central portion, and an inner surface to engage an edge of the substrate. The inner surface has an adjustable diameter. The controller is configured to, in response to identifying a polishing non-uniformity decrease the diameter of the inner surface of the retainer sufficiently that the substrate bows, determine whether the substrate should bow inwardly or outwardly from the carrier head to reduce the polishing non-uniformity, and select whether the first pressure is greater or lower than the second pressure such that the substrate bows in the determined direction.

Implementations may optionally include, but are not limited to, one or more of the following advantages. Distribution of force during polishing between the substrate and the retaining ring can be modified so that force is redistributed along the edge of the substrate. This distributed contact force can reduce local wafer deformations and can improve the operator's ability to control substrate edge removal profile. Polishing non-uniformity, e.g., caused by a polishing head profile issue at a substrate edge, can be reduced. The retaining ring can be operated at higher clamping force in concert with the pressure from the membrane of the carrier head to change the shape of the substrate, which can modify polishing rates across the substrate.

The details of one or more implementations are set forth in the accompanying drawings and the description below. Other aspects, features, and advantages will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a carrier head in a chemical mechanical polishing system.

FIG. 2A shows a cross-sectional side view of the retainer assembly of FIG. 1 .

FIG. 2B shows a bottom view of the retainer assembly of FIG. 2A.

FIG. 2C shows a cross-sectional side view of the retainer assembly of FIG. 2A in an actuated configuration.

FIG. 3 shows a cross-sectional side view of another implementation of a retainer assembly.

FIG. 4 shows a cross-sectional side view of yet another implementation of a retainer assembly.

FIG. 5 shows a cross-sectional side view of still another implementation of a retainer assembly.

FIGS. 6A and 6B are schematic illustrations showing pressures applied to bow a substrate into a convex or concave configuration, respectively.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

Some carrier heads include a retaining ring with a circular inner surface to retain the substrate. Typically the inner diameter of the retaining ring is slightly larger than the diameter of the substrate, e.g., by 1-3 mm. In this configuration the substrate can rotate relative to the carrier head and retaining ring; this relative movement is termed “precession.” Precession can be useful for reducing asymmetric non-uniformities.

During polishing frictional forces can drive the substrate edge against the inner surface of the retaining ring. A potential problem with a circular inner surface is that the force from the substrate can be concentrated at a single point of contact between the substrate and the retaining ring, which can lead to scratching or other damage to the inner surface, or to unintended warping of the substrate near the point of contact, which can induce polishing non-uniformities.

A retaining ring with a flexible inner surface or with an adjustable inner diameter has been proposed. Hypothetically such a configuration would permit the retaining ring diameter to be reduced so that the substrate contacts along an extended region rather than at a single point. However, such a configuration has apparently not been commercialized. Thus, there remains room for improvement on design of a retaining ring having an adjustable inner diameter.

FIG. 1 illustrates an example of a polishing station of a chemical mechanical polishing system 20. The polishing system 20 includes a rotatable disk-shaped platen 24 on which a polishing pad 30 is situated. The platen 24 is operable to rotate about an axis 25. For example, a motor 26 can turn a drive shaft 28 to rotate the platen 24. The polishing pad can be a two-layer polishing pad with an outer polishing layer 32 and a softer backing layer 34.

The polishing system 20 can include a supply port or a combined supply-rinse arm 36 to dispense a polishing liquid 38, such as an abrasive slurry, onto the polishing pad 30. The polishing system 20 can include a pad conditioner apparatus 40 with a conditioning disk 42 to maintain the surface roughness of the polishing pad 30. The conditioning disk 42 can be positioned at the end of an arm 44 that can swing so as to sweep the disk 42 radially across the polishing pad 30.

A carrier head 70 is operable to hold a substrate 10 against the polishing pad 30. The carrier head 70 is suspended from a support structure 50, e.g., a carousel or a track, and is connected by a drive shaft 54 to a carrier head rotation motor 56 so that the carrier head can rotate about an axis 58. Optionally, the carrier head 70 can oscillate laterally, e.g., on sliders on the carousel, by movement along the track, or by rotational oscillation of the carousel itself.

The carrier head 70 includes a housing 72, a substrate backing assembly 74 which includes a base 76 and a flexible membrane 78 that defines a plurality of pressurizable chambers 80, a gimbal mechanism 82 (which may be considered part of the assembly 74), a loading chamber 84, a retaining ring assembly 100, and an actuator 122.

The housing 72 can generally be circular in shape and can be connected to the drive shaft 54 to rotate therewith during polishing. There may be passages (not illustrated) extending through the housing 72 for pneumatic control of the carrier head 70. The substrate backing assembly 74 is a vertically movable assembly located beneath the housing 72. The gimbal mechanism 82 permits the base 76 to gimbal relative to the housing 72 while preventing lateral motion of the base 76 relative to the housing 72. The loading chamber 84 is located between the housing 72 and the base 76 to apply a load, i.e., a downward pressure or weight, to the base 76 and thus to the substrate backing assembly. The vertical position of the substrate backing assembly 74 relative to a polishing pad is also controlled by the loading chamber 84. The lower surface of the flexible membrane 78 provides a mounting surface for a substrate 10.

In some implementation, the substrate backing assembly 74 is not a separate component that is movable relative to the housing 72. In this case, the chamber 84 and gimbal 82 are unnecessary.

Referring now to FIGS. 1 and 2A, the retaining ring assembly 100 includes an inner ring 110, a first actuator 120 to adjust a vertical height of or downward pressure on the inner ring 110, an outer ring 130, and a second actuator 140 between the inner ring 110 and the outer ring 130 to adjust a radially inward directed pressure on the inner ring 110. A lower surface 112 of the inner ring 110 can contact the polishing pad 30. Similarly, a lower surface 132 of the outer ring 130 can contact the polishing pad 30.

The inner ring 110 is an annular body that is vertically movable relative to the housing 72. The inner ring 110 has an inner surface that is configured to circumferentially surround the edge of the substrate 10 to retain the substrate 10 in the carrier head during polishing. The inner surface of the inner ring 110 can be a vertical cylindrical surface that extends from the lower surface 112 to the upper annular surface.

An outer surface of the inner ring 110 can optionally include a lip 114 at the lower surface 112 that projects outwardly from a cylindrical portion toward the outer ring 130. The lip 114 can abut the outer ring 130 to restrain movement of the inner ring 110 without inducing significant torque out of the plane of the polishing surface. In some implementations, the inner ring includes a lower portion 110 b formed of a wearable material, e.g., a plastic, and an upper portion 110 a formed of a more rigid material, e.g., a metal.

Referring to FIG. 2B, the inner ring 110 includes multiple radial slots 116 that extend from the bottom surface 112 upward past the actuator 140. Thus the inner ring 110 is divided by into multiple arc segments 118 a that extend downwardly from a circular upper portion 118 b. As discussed below, these arc segments 118 a are independently flexible relative to the connecting circular upper ring 118 b. Each arc segment 118 a can extend through an arc (relative to the center of the inner ring 110) of 5-20°. The slots 116 can be uniformly spaced at equal angular intervals around the inner ring 110.

Returning to FIG. 2A, the first actuator 120 can be a pressuizable chamber. For example, an annular membrane 122 can have an outer lip clamped to the housing 72 and an inner lip clamped to a top surface of the inner ring 110 to form a chamber 126. Alternatively, the first actuator can be provided by an inflatable bladder, or by a linear motor or piezoelectric actuator.

The outer ring 130 is vertically fixed relative to the housing 72, and is an annular body that provides positioning or referencing of the carrier head 70 to the surface of the polishing pad 30. In addition, the outer ring 130 provides lateral referencing of the inner ring 110 against the polishing pad 30. The outer ring 130 circumferentially surrounds, e.g., is coaxial with, the inner ring 110.

The outer ring 130 has an outer surface, which can be a vertically cylindrical surface. The vertical cylindrical outer surface can extend upwardly from an outer edge of the lower surface 132. The outer ring 130 also has an inner surface that is separated by a gap 134 from the outer surface of the inner ring 110. In some implementations, the outer ring includes a lower portion 130 b formed of a wearable material, e.g., a plastic, and an upper portion 130 a formed of a more rigid material, e.g., a metal. In some implementations, the entirety of the inner ring 110 is formed of a material that is more flexible than the upper portion 130 a of the outer ring. For example, the inner ring 110 can be formed of the same material as the lower portion 130 b of the outer ring 130.

The outer ring 130 can be secured to the housing 72, for example, by an adhesive, a fastener, or by interlocking parts. For example, an upper surface 136 of the outer ring 130 can include cylindrical recesses or holes with screw sheaths (not shown) to receive fasteners, such as bolts, screws, or other hardware. For example, a fastener, such as a screw or bolt, can extend through the housing 72 to secure the outer ring 130 of the retaining ring assembly 100 to the housing 72.

The inner ring 110 can be relatively narrow as compared to the outer ring 130. For example, the inner ring 110 can have a width W of 1-10 mm, e.g., 1-3 mm, e.g., 2 mm. The width W can be measured at the narrow section of the inner ring 1110, e.g., above the lip 114.

The second actuator 140 is positioned between the inner surface of the outer ring 130 and the outer surface of the inner ring 110. The second actuator 140 can be an inflatable annular bladder; pressurization of the bladder inflates the bladder and exerts a radially inwardly directed pressure on the arc segments 118 a of the inner ring 110. Alternatively, the second actuator can be provided by an inflatable bladder, or by a linear motor or piezoelectric actuator. In any event, as shown in FIG. 2C, actuation by the second actuator 140 causes the arc segments 118 a to flex inwardly relative to the upper ring portion 118 b. As a result, the effective diameter of the inner surface of the inner ring 110 that contacts the substrate is reduced. Sufficient reduction of the diameter, e.g., by 1-3 mm, can cause the inner ring 118 to “clamp” the substrate, i.e., establish contact along the entire circumference of the substrate.

For pneumatic control of the second actuator 140, a pneumatic control line 92 can extend from the bladder to a controllable pressure source 94. The control line 92 can be provided by a combination of passages through solid parts, piping, tubing, etc. The control line 92 can extend through the housing 72, and the drive shaft 54, and be connected to the pressure source 92 by a rotary coupling.

FIG. 2A illustrates that the second actuator 140 fits at least partially into a recess 138 in the inner surface of the outer ring 130. However, there could be a recess in the outer surface of the inner ring, or both rings could have aligned recesses, to accommodate the second actuator 140.

The arc segments 118 a of the inner ring 110 can flex so the portion adjacent the lower surface 112 is horizontally movable relative to the outer section 144 when acted upon by the second actuator 140. In particular, when the second actuator 140 presses inwardly, the arc segments 118 a flex inwardly, so the gaps between the arc segments narrow and the effective diameter of the inner surface of the inner ring 110 decreases.

Returning to FIG. 1 , inward pressure on the arc segments 118 a of the inner ring 110 by the second actuator 140 is controlled by a controller 90. For example, the controller 90 can control the pressure applied by the pressure source 94, e.g., by controlling a valve or the like. The controller 90 can also control downward pressure on the inner ring 110 by the first actuator 120, as well as other polishing system parameters, e.g., carrier head rotation rate, platen rotation rate, pressure of chamber inside the carrier head, slurry dispensing rate, etc. The controller 90 can be provided by a dedicated circuitry, a general purpose or programmable computer or application specific integrated circuit that executes instructions stored in a non-transitory computer readable medium.

FIG. 3 illustrates an implementation of the retaining ring assembly 100 which is similar to that of FIG. 2A, but the inner ring 110 and outer ring 130 are secured so as to be vertically fixed relative to each other. The inner ring 110 and outer ring 130 together can be considered to form a combined retainer 150. The combined retainer 150 can either be vertically movable relative to the housing 72 by the first actuator 120, e.g., as described above for the inner ring in FIG. 2A, or vertically fixed to the housing 72, e.g., by adhesive, mechanical fasteners, etc., as described above for the outer ring in FIG. 2A (in this case, there is no “first actuator”).

In some implementations, the inner ring 110 includes an outwardly extending flange 152 that extends over the top surface of the outer ring 130. The bottom of the flange 152 can be secured to the top surface of the outer ring 130, e.g., by adhesive, mechanical fasteners, etc.

In an unbiased state, the portions of the outer surface of the inner ring 110 and the inner surface of the outer ring 130 located above the second actuator 140 are separated by a vertical cylindrical gap 134. The gap 134 can be relatively narrow, e.g., 10-100 μm. FIG. 4 illustrates an implementation of the retaining ring assembly 100 which is similar to that of FIG. 3 , but instead of the inner ring 110 having an outwardly extending flange, the outer ring 130 has an inwardly extending flange 160 that extends over the top surface of the inner ring 110. The bottom of the flange 160 can be secured to the top surface of the inner ring 110, e.g., by adhesive, mechanical fasteners, etc.

FIG. 5 illustrates an implementation of the retaining ring assembly 100 which is similar to that of FIGS. 3-4 , but the second actuator 140 is positioned to apply both an inward force on the outer surface of the inner ring 110, and a downward force on the upper surface of the lip 114. This downward force will also tend to cause the arc segments 118 a to flex inwardly.

By reducing the effective diameter of the inner ring, and in particular by reducing the effective diameter until the substrate is clamped, lateral force of the substrate on the retainer is distributed across a significant arc rather at a single point. This distributed contact force between can reduce local wafer deformations, and can reduce the likelihood of scratching and damage to the inner surface of the retainer. More generally, the inward pressure provides another “knob” to adjust the polishing profile, permitting greater flexibility and ability to control the substrate edge removal profile.

A potential danger with clamping the substrate is that clamping can prevent the substrate from precessing relative to the carrier head. However, precession can reduce asymmetric (i.e., angularly varying) polishing non-uniformities. Thus, the controller 90 can be configured to operate the second actuator so that the substrate is temporarily released from clamping (but still retained) to allow precession and then clamped again. In other words the controller can, while the substrate is being polished, cause the retaining ring assembly to alternate between a first pressure at which the substrate is clamped and a second pressure at which the substrate is released and free to precess.

Referring to FIGS. 6A and 6B, the retaining ring assembly 100 can be operated in conjunction with the pressurizable chambers 80 of the carrier head 70 to deliberately change the shape of the substrate, e.g., to cause the substrate to assume a convex (center bowing outward toward the polishing pad) or a concave (center bowing inward away from the polishing pad) configuration.

Referring to FIG. 6A, an inward clamping force (A) on the substrate edge will tend to cause the substrate to bow. If the downward pressure (B) applied to the center of the substrate is greater than the downward pressure (C) applied to the edges of the substrate (e.g., if the pressure in a center chamber is higher than a pressure in a surrounding outer chamber), then the substrate will tend to assume a convex configuration. This will cause the pressure of the polishing pad on the center of the substrate to increase, so the center polishing rate will increase relative to edge polish rate.

In contrast, referring to FIG. 6B, if the downward pressure (B) applied to the center of the substrate is less than the downward pressure (C) applied to the edges of the substrate (e.g., if the pressure in a center chamber is lower than a pressure in a surrounding outer chamber), then the substrate will tend to assume a concave configuration. This will cause the pressure of the polishing pad on the center of the substrate to decrease, so the center polishing rate will decrease relative to edge polish rate.

The controller 90 can be configured to cause the carrier head to apply appropriate pressures to the substrate so as to selectively cause the substrate to assume a concave or convex configuration. For example, if the controller 90 receives data from an in-situ monitoring system and detects that the substrate edge is polishing faster than the substrate center, the controller 90 can cause the downward pressure (B) applied to the center of the substrate to be greater than the downward pressure (C) applied to the substrate edge so the center polishing rate will increase relative to edge polish rate.

As used in the instant specification, the term substrate can include, for example, a product substrate (e.g., which includes multiple memory or processor dies), a test substrate, a bare substrate, and a gating substrate. The substrate can be at various stages of integrated circuit fabrication, e.g., the substrate can be a bare wafer, or it can include one or more deposited and/or patterned layers. The term substrate can include circular disks and rectangular sheets.

The above described polishing system and methods can be applied in a variety of polishing systems. Either the polishing pad, or the carrier head, or both can move to provide relative motion between the polishing surface and the substrate. The polishing pad can be a circular (or some other shape) pad secured to the platen. The polishing layer can be a standard (for example, polyurethane with or without fillers) polishing material, a soft material, or a fixed-abrasive material. Terms of relative positioning are used; it should be understood that the polishing surface and substrate can be held in a vertical orientation or some other orientation.

Particular embodiments of the invention have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results. 

What is claimed is:
 1. A method of polishing, comprising: bringing a substrate into contact with a polishing pad and generating relative motion between the substrate and the polishing pad; retaining the substrate on the polishing pad with a retainer; and during polishing of the substrate, alternating between reducing a diameter of an inner surface of the retainer to clamp the substrate and increasing the diameter of the inner surface of the retainer to release the substrate from clamping while continuing to retain the substrate.
 2. The method of claim 1, wherein the substrate precesses relative to the retainer while the substrate is released from clamping by the retainer.
 3. The method of claim 1, wherein the substrate has a fixed angular orientation relative to the retainer while the substrate is clamped by the retainer.
 4. The method of claim 1, comprising alternating a plurality of times during polishing of the substrate.
 5. The method of claim 1, wherein reducing a diameter of the inner surface comprising applying inward pressure to a plurality of arcuate segments of an inner ring.
 6. A polishing system, comprising: a support to hold a polishing pad; a carrier head to hold the substrate against the polishing pad, the carrier head including a retainer having an inner surface to engage an edge of the substrate and wherein the inner surface has an adjustable diameter; a controller configured to cause an actuator to alternate during polishing of the substrate between reducing a diameter of the inner surface of the retainer to clamp the substrate and increasing the diameter of the inner surface of the retainer to release the substrate from clamping while continuing to retain the substrate.
 7. The system of claim 6, wherein the controller is configured to cause the actuator to alternate a plurality of times during polishing of the substrate.
 8. The system of claim 6, wherein the retainer includes and inner ring having a plurality of arcuate segments and an outer ring.
 9. The system of claim 8, wherein the actuator is positioned between the inner ring and the outer ring.
 10. The system of claim 9, wherein the actuator comprises a bladder.
 11. A polishing system, comprising: a support to hold a polishing pad; a carrier head to hold the substrate against the polishing pad, the carrier head including a first chamber to apply a first downward pressure to a center portion of the substrate held by carrier head, a second chamber to apply a second downward pressure to an outer portion of the substrate surrounding the central portion, and a retaining ring having an inner surface to engage an edge of the substrate and wherein the inner surface has an adjustable diameter; a controller configured to, in response to identifying a polishing non-uniformity, decrease the diameter of the inner surface of the retainer sufficiently that the substrate bows, determine whether the substrate should bow inwardly or outwardly from the carrier head to reduce the polishing non-uniformity, and select whether the first pressure is greater or lower than the second pressure such that the substrate bows in the determined direction.
 12. The polishing system of claim 11, wherein the controller is configured to, in response to detecting that a center portion of the substrate is overpolished relative to an edge portion of the substrate, cause the first pressure to be less than the second pressure.
 13. The polishing system of claim 12, wherein the controller is configured to decrease the diameter of the inner surface of the retainer sufficiently that the substrate bows to form a convex shape.
 14. The polishing system of claim 11, wherein the controller is configured to, in response to detecting that a center portion of the substrate is underpolished relative to an edge portion of the substrate, cause the first pressure to be larger than the second pressure.
 15. The polishing system of claim 14, wherein the controller is configured to decrease the diameter of the inner surface of the retainer sufficiently that the substrate bows to form a concave shape.
 16. The polishing system of claim 11, wherein the retaining ring includes and inner ring having a plurality of arcuate segments and an outer ring.
 17. The polishing system of claim 16, wherein the actuator is positioned between the inner ring and the outer ring.
 18. The polishing system of claim 17, wherein the actuator comprises a bladder. 